Aortic aneurysms represent a significant medical problem for the general population. Aneurysms within the aorta presently affect between two and seven percent of the general population and the rate of incidence appears to be increasing. This form of vascular disease is characterized by a degradation in the arterial wall in which the wall weakens and balloons outward by thinning. If untreated, the aneurysm can rupture resulting in death within a short time.
The traditional treatment for patients with an abdominal aortic aneurysm is surgical repair. This is an extensive operation involving transperitoneal or retroperitoneal dissection of the aorta and replacement of the aneurysm with an artificial artery known as a prosthetic graft. This procedure requires exposure of the aorta through an abdominal incision extending from the lower border from the breast bone down to the pubic bone. The aorta is clamped both above and below the aneurysm so that the aneurysm can be opened and the prosthetic graft of approximately the same size as the aorta is sutured in place. Blood flow is then re-established through the prosthetic graft. The operation requires a general anesthesia with a breathing tube, extensive intensive care unit monitoring in the immediate post-operative period along with blood transfusions and stomach and bladder tubes. All of this imposes stress on the cardiovascular system. This is a high-risk surgical procedure with well-recognized morbidity and mortality.
More recently, significantly less invasive clinical approaches to aneurysm repair known as endovascular grafting have been proposed. (See, Parodi, J. C., et al. "Transfemoral Intraluminal Graft Implantation for Abdominal Aortic Aneurysms," 5 Annals of Vascular Surgery, 491 (1991)). Endovascular grafting involves the transluminal placement of a prosthetic arterial graft in the endoluminal position (within the lumen of the artery). By this method, the graft is attached to the internal surface of an arterial wall by means of attachment devices such as expandable stents, one above the aneurysm and a second below the aneurysm.
Although endovascular grafting represents a desirable improvement over traditional surgical repair, current endovascular graft systems suffer from certain deficiencies. For example, current endovascular graft systems typically are unsuitable for use in an aneurysm which is tortuous. Aneurysms in the aorta create tortuosity as they grow. Aneurysms grow both in diameter and length, thus "pushing" the adjacent upper and lower portions of the arteries upward and downward, respectively. Since the aorta is relatively "fixed" at the renal arteries, the portion of the aorta below and near the renal arteries becomes bent and curved in order to accommodate the added length. A similar phenomenon occurs below the aneurysm in the iliac arteries, leading to tortuous iliacs. As many as 20% of aortic aneurysms may have so much tortuosity that they are unable to be fitted with an endovascular graft of this kind. Such systems are unable to conform to the curved walls of the vasculature due to the tortuosity caused by the growing aneurysm.
A specific problem is the "angulation" or bend in the neck of the aorta, where it meets the upper part of the aneurysm. This angulation may result in several problems which limit the effectiveness of traditional endovascular graft systems which do not have designs that conform to the tortuosity and angulation above the aneurysm. First, since these systems are typically anchored above the aneurysm with a stent, a portion of the stent may extend into the blood flow path, creating turbulence which may result in blood clotting. It is well-known that in coronary vessels, stents used to treat constrictive lesions must be well apposed to the wall of the vessel to prevent the possibility of thrombosis. Second, a non-conforming upper stent will not place the upper end of the graft in good apposition to the aortic wall, making it difficult to obtain a good seal with a conventional endovascular graft system. Such is illustrated in FIG. 2, showing a generic endovascular graft attached to a conventional non-conforming expanded metal stent in the neck of a tortuous aortic neck. Since this conventional stent will not conform to the tortuosity of the aorta, an upper edge 1 of the stent extends into the blood flow path increasing the chance of thrombosis. Further, a lower edge 2 is not apposed to the wall of the aorta so that the graft material 3 affixed to it does not properly seal. A third problem with nonconforming attachment systems is that once placed in tortuous or angulated aneurysmal anatomy, they are unstable and can "pop-out" of position. The attachment system shown in FIG. 2 is an example of an unstable attachment system. Conventional endovascular graft systems having an attachment system intended to project across and above the renal artery ostia also pose a different problem since the attachment system obstructs the renal arteries making it difficult, if not impossible, to effect a repair on a renal artery once the stent is in place.
Thus, a need exists for a prosthetic endovascular graft system which will permit stable conformance to bends within an aneurysm, while providing a good seal to the vasculature.
Another challenge for endovascular grafting of aortic aneurysms relates to the need for graft systems to be delivered in as small of a "profile" as possible. This has driven the design of most endovascular grafting systems to be fabricated with very thin-walled graft conduits. This thin walled conduit, usually coupled to an internal support framework, typically a metallic framework attached to the graft conduit on either the inside or on the outside, is susceptible to "wear and tear" mechanisms arising from the pulsatile blood pressure and flow in the aorta. Numerous incidences have been reported in the literature of holes and tears being created in the graft conduit from the cyclic, localized rubbing of the metallic framework against the thin walled graft conduits of a variety of endovascular grafting systems.
Thus, a need exists for a prosthetic endovascular graft system which will minimize or eliminate the wearing mechanisms on the tubular graft conduit, enabling the graft system to be safely utilized in patients for long periods of time, i.e. several years, without concern of premature failure due to wear.
Yet another concern of current endovascular graft systems relates more specifically to the long-term integrity of metallic stents which are used for supporting the structure of the graft material. Since portions of many of the stents used for graft support are in direct interface with the aorta (and iliac arteries in the case of biluminal endovascular graft systems), the pulsatile forces that cause pulsatile diameter changes on these vessels are transferred to these stent portions. This pulsation in the stents leads to cyclic stressing, and can cause premature fatigue failure and breakage.
Thus, a need exists for a prosthetic endovascular graft system which incorporates stents that are designed to minimize cyclic stresses and thus avoid fatigue failure.